Optical absorption, PDAs

Figure 13 Optical absorption of the PDA poly-4BCMU in three differently ordered states (a) solution in chloroform (b) gel from toluene (dense homogeneous films cast from solution have similar, though slightly less structured, spectra) (c) single crystal.

The optical absorption maxima are revealed to be at 520, 500 and 420 nm for PTV, MOPPV and PPV films, respectively. X-ray diffraction patterns indicate that the PAV films have low crystallinity. The values of these PAV films evaluated by third-harmonic generation (THG) measurement at 1.85 pm are found to be 5.85 x 10 " esu, 3.2 X 10 esu, and 7.8 x 10 esu for PTV, MOPPV and PPV films, respectively. These values are non resonant values and are almost the same as that reported for a processible polydiacetylene (n-butoxycarbonylmethyl urethane) (n-BCMU-PDA, n = 4,3) thin film [18]. 

Fig. 5 Conparison of the relative optical absorption of a PDA.-TS crystal obtained by transBdssion with Eib (solid line) and the absorption coefficient determined by Kramers-Kronig aneU.ysis of reflectivity data with ENb (dashed line).

Phase transitions have also been observed in PDA mono- and multilayer LB films. The optical absorption data in Fig. 15 illustrate the irreversible hase transition idiich occurs with increasing teBqg>erature in a partially polymerised multilayer sanqple of the cadmium salt of PDA-12/BAFA (49). The same transition can also be induced by washing the sanqple with ethanol (50), or exposing the sample for longer times in the UV source used to effect polymerisation (51). The initial polymer forms with peak A at approximately 1.9 eV while in the final phase the peak has shifted, to 2.3 ev. The spectra for both phases are characteristic of partially disordered crystalline material. Sufficient electron... 

Why is the photoinduced electron transfer so strongly inhibited in the polydiacetylenes The energetics of the donor and acceptor components and the relative positions of the various energy levels are important parameters of the photoinduced electron transfer mechanism. The ionization potentials of the polydiacetylenes are around 5.5 eV [170], nearly identical to the ionization potentials of polythiophenes (5.2 eV [171]) and very close to those of the PPVs (5.11 eV [171]). The optical absorption gaps are comparable. Thus, the extraordinary difference— ultrafast photoinduced electron transfer in the PPVs and P3ATs and the complete inhibition of photoinduced electron transfer in the PDAs— must have its origin in the photophysics of the polydiacetylene. 

Figures 21-23 show, respectively the optical absorption spectrum, frequency variation of x (-3w c ),(o,a)) and of x M"2a) (i),(o,0) cubic susceptibilities determined by THG and EFISH for a PDA thin film. In all these spectra one observes vibronic side bands visualized by shoulders in the smaller wavelength range with respect to. These vibronic side...

The %(3) values were almost the same for PPV, PTV and MOPPV in the absorption wavelength region. Dispersion measurements are required in this energy range in order to clarify the nature of the resonance and to determine the maximum resonantly enhanced %0). The evaluated susceptibilities of the PAV films are almost the same order of magnitude as that of PDA films. This result indicates that PAV thin films may be effective for NLO device application, due to the ease in processing films as well as possible good optical quality. 

All real CP samples except PDAs are disordered, as shown in Section II. Disorder is usually associated with band tails, that is, energy levels extending from the band edge into the gap and corresponding to states localized in space. That this is so in CPs as well is shown by the absorption tails extending below the optical gap [130]. These states will influence the motion of carriers. 

The nonlinear optical properties in solution of selected functionalized PDAs, described herein, have also been evaluated by means of the z-scan technique. Off resonance studies (at 705 nm) show that nonlinear refraction is only comparable to that of the solvent for these dilute solutions, but that nonlinear absorption, characterized by p values, varies significantly, with the nature of the side-chains. O It can be inferred that if bulk films of these PDAs possess suitable nonresonant nonlinear refractive properties for optical devices, modification of side-chain structure can reduce the magnitude of undesirable two photon absorption. 

Near resonant z-scan studies provide both nonlinear refractive coefficients, ni, and two photon absorption coefficients, p. Results for a selection of PDAs in solution at 532 nm are given in Table 1. Diethylamino-PDA (lid) and the related quatemized polymer (15a) show comparable magnitudes of nonlinear absorption and refraction to those of poly-4-BCMU, with figures of merit, T, which satisfy the requirement for nonlinear optical switching devices. The metal-containing PDAs... 

At 532 nm, dilute solutions of potassium salts of urethane carboxylic acid PDAs, [=RC-CsC-CR=]n, R = (CH2)m02CNHCH2C02-K+, m = 3, 4 show resonant optical nonlinearities dominated by a saturable absorption effect, with Int(x )... 

Microcrystals of some diacetylenes, prepared by the reprecipitation method, have been studied as dispersions in liquid media. Interesting behavior has been observed in the solid-state polymerization of diacetylene monomers and with the optical properties of polydiacetylene (PDA) microcrystals. First, the polymerization perfectly proceeded from one end to the other end of the diacetylene microcrystals. Next, the excitonic absorption peak position was found to shift to higher energy side with decreasing size of the PDA microcrystals. The size effect was observed even for crystals as large as 100 nm or more in contrast to conventional quantum effect of inorganic semiconductors where size effect is observed only for microciystals of less than about 10 nm size. In addition, since the microcrystal dispersions in water have low optical loss, the c tical Kerr shutter response of PDA microciystals could be measured, and the non-resonant value was estimated to be on the order of 10 to 10" esu in very low concentrations (ca. 10 M). 

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